In observing a sample, such as a sliced cell and a nerve cell in vivo, an observation method which has been available captures a plurality of images of an observation cross-section on an XY plane, which is perpendicular to the optical axis, while changing the Z coordinate along the optical axis direction, and constructs a three-dimensional image of a rectangular parallelopiped area inside the sample by the image processing (volume rendering) to stack the plurality of images on the observation cross-section in the Z direction, so as to perform a stereoscopic observation of the sample. In such an observation method, the user determines a range of the three-dimensional image in the Z direction (Z stack), while observing the image of the observation cross-section on the XY plane in real-time, while adjusting the Z coordinate, for example.
For example, Patent Document 1 discloses an observation method for obtaining a three-dimensional image of a sample, specifying an area to execute light stimulation based on the three-dimensional image, and observing the change of the sample caused by the light stimulation.
A nerve cell has a structure of a nerve cell body from which dendrites extend, and depending on the orientation of the sample in the observation, dendrites are positioned in a vertical direction or in a downward diagonal direction. In this case, even if the user observes the image of the observation cross-section in the XY plane in real-time, dendrites are displayed merely as dots in each observation cross-section in an individual image, and it is difficult to know the general structure of the nerve cell merely by obtaining the images of the observation cross-sections.
To solve this problem, Patent Documents 2 and 3, example, disclose an observation method for grasping a general structure of an entire sample by obtaining a three-dimensional image thereof, specifying an observation area along the dendrites based on the three-dimensional image, and observing the observation area closely.    Patent Document 1: Japanese Patent Application Laid-Open No. 2006-3805    Patent Document 2: Japanese Patent Application Laid-Open No. 2005-24642    Patent Document 3: Japanese Patent Application Laid-Open No. 2003-195174
In the case of the above mentioned observation method for constructing a three-dimensional image of a rectangular parallelopiped area inside a sample to observe the sample, it takes time to execute the processing to capture images on a plurality of observation cross-sections and the processing to stack these images in the Z direction, therefore efficient observation is difficult.